The present invention relates to a lithographic printing plate precursor and, more particularly, to a direct-image type lithographic printing plate precursor which is suitably used to make printing plates for office work purposes.
Now, a direct-image type lithographic printing plate precursors having an image-receiving layer on a support have been widely used to make printing plates for office work purposes. For making printing plates with such precursors or in order to form images on them, the images are handwritten on them with oil ink or formed on them as by typewriters, ink-jet printing or transfer type thermal printing. Recently, there is also available a technique in which a toner image is formed on a photosensitive material through the steps of charging, exposure and development carried out using a plain paper electrophotographic copy machine (PPC) and then transferred and fixed onto an image-receiving layer. In order to enable this precursor to be used as a printing plate for lithography, in any case, it should be treated on its surface with a desensitizing (or etching) solution to desensitize the non-image area.
The conventional direct-image type lithographic printing plate precursor has comprised a support made up of a paper sheet, on both sides of which a back layer and a front layer are applied, the latter being provided through an interlayer. The back layer or interlayer is made f a water-soluble resin such as PVA starch, a water-dispersible resin such as a synthetic resin emulsion and a pigment. The front layer is made up of a pigment, a water-soluble resin and a waterproofing agent.
As typically set forth in U.S. Pat. No. 2,532,865 specification, such a direct-image type lithographic printing plate precursor has an image-receiving layer composed mainly of a water-soluble resin binder such as PVA, an inorganic pigment such as silica or calcium carbonate and a waterproofing agent such an initial melamine-formaldehyde condensate.
Further, the binder used for the image-receiving layer of the direct-image type lithographic printing plate precursor is pre-crosslinked, containing a functional group capable of forming a carboxyl group, a hydroxyl or thiol group, an amino group, a sulfone group and a phosphono group upon decomposition and a functional group set by heat/light (Japanese Patent Application Nos. 63-54609 and 63-117035 and Japanese Provisional Publication No. 1-269593). It is also proposed to use the binder in combination with thermosetting/photosetting resins (see Japanese Provisional Patent Publication Nos. 1-266546 and 1-275191 as well as Japanese Patent Application No. 63-139344) or in combination with crosslinkers (see Japanese Provisional Patent Publication Nos. 1-267093, 1-271292 and 1-309067), thereby improving on not only the hydrophilic nature of the non-image area and the film strength of the image-receving layer but also plate wear as well.
However, a problem with the printing plate precursors so obtained is that when an increased quantity of a waterproofing agent, or a hydrophobic resin, is used to enhance their hydrophobic nature for the purpose of increasing their printing serviceability, there is an increase in their plate wear but there is a decrease in their hydrophilic nature, which would otherise result in scumming, whereas improving on their hydrophilic nature makes them poor in water resistance and plate wear. A particularly grave problem with them is that when they are used at a high temperature exceeding 30.degree. C., their surface layers are dissolved in the dampening water used for offset printing, giving rise to a drop of plate wear and scumming.
Another problem with the lithographic printing plate precursors, on the image-receiving layers or image areas of which images are formed with oil ink, is that if the receiving layers do not show well adhesion to oil ink, then the oil ink peels away from the image areas during printing, resulting in a drop of plate wear. This is true even when the non-image areas have hydrophilic nature enough to prevent scumming.
The present invention has been achieved with a view to eliminating the above problems with a conventional direct-image type of lithographic printing plate precursors.
One object of this invention is to provide a direct-image type lithographic printing plate precursor which can be well desensitized and so can be used as an offset printing plate precursor free from not only overall uniform scumming but a spot-form of scumming as well.
Another object of this invention is to provide a lithographic printing plate precursor in which oil ink on the image area has an improved adhesion to the image-receiving layer and the hydrophilic nature of the non-image area is well retained even after printing is repeated over and over, and which has a high plate wear and does not give rise to scumming.